For grease lubricating oils, engine oils, gear oils, hydraulic oils, etc., various properties are required, and in recent years, as the internal combustion engines are made to have higher performance and higher output power or as the operation conditions become severer, higher properties of internal combustion engine parts, such as higher abrasion resistance, heat resistance, sludge resistance, lubricating oil consumption properties and fuel saving, have been required. In particular, long drain interval (lengthening of life) has been required, and for the requirement, evaporation loss of the lubricating oils and lowering of viscosity attributable to shearing need to be further reduced. On the other hand, in order to ensure fuel saving, wide ranging has been promoted, and excellent low-temperature viscosity properties of the lubricating oils have become necessary. Accordingly, as the grease lubricating oils, engine oils, gear oils, hydraulic oils, etc., those having long life, namely, those having low-evaporation properties and favorable temperature-viscosity properties, have been desired.
As synthetic lubricating oils for use as lubricating base oils of automotive gear oils, engine oils, industrial lubricating oils and hydraulic oils, poly-α-olefins (PAO) have been largely used industrially. Such PAO can be obtained by oligomerization of higher α-olefins using acid catalysts, as described in U.S. Pat. No. 3,780,128, U.S. Pat. No. 4,032,591 and Japanese Patent Laid-Open Publication No. 163136/1989.
On the other hand, it is known that ethylene/α-olefin copolymers are also employable as synthetic lubricating oils having excellent viscosity index, oxidation stability, shear stability and heat resistance, as described in Japanese Patent Laid-Open Publication No. 117595/1982.
For preparing the ethylene/α-olefin copolymers used as the synthetic lubricating oils, such a process using a vanadium catalyst comprising a vanadium compound and an organoaluminum compound as described in Japanese Patent Publication No. 1163/1990 and Japanese Patent Publication No. 7998/1990 has been heretofore employed. Of the ethylene/α-olefin copolymers, an ethylene/propylene copolymer is mainly employed.
As a process for preparing copolymers with high polymerization activity, such a process using a catalyst system comprising a metallocene compound such as zirconocene and an organoaluminum oxy-compound (aluminoxane) as described in Japanese Patent Laid-Open Publication No. 221207/1986 and Japanese Patent Publication No. 121969/1995 is known, and in Japanese Patent No. 2796376, a process for preparing a synthetic lubricating oil comprising an ethylene/α-olefin copolymer that is obtained by the use of a catalyst system comprising a combination of a specific metallocene catalyst and aluminoxane is disclosed.
In recent years, the environment where the lubricating oils are used is becoming severer, while life lengthening of lubricating oils tends to be desired in consideration of environmental problems, and the demand for the synthetic lubricating oils such as PAO and an ethylene/propylene copolymer having excellent low-temperature viscosity properties and thermal oxidation stability tends to be increased. From the viewpoints of reduction of fuel consumption and saving of energy, further improvements in viscosity index and low-temperature viscosity properties have been desired.
Patent document 1: U.S. Pat. No. 3,780,128
Patent document 2: U.S. Pat. No. 4,032,591
Patent document 3: Japanese Patent Laid-Open Publication No. 163136/1989
Patent document 4: Japanese Patent Laid-Open Publication No. 117595/1982
Patent document 5: Japanese Patent Publication No. 1163/1990
Patent document 6: Japanese Patent Publication No. 7998/1990
Patent document 7: Japanese Patent Laid-Open Publication No. 221207/1986
Patent document 8: Japanese Patent Publication No. 121969/1995
Patent document 9: Japanese Patent No. 2796376